Question

In the C4 pathway, which enzyme is responsible for the primary carboxylation in mesophyll cells?

• A. RuBisCO
• B. PEP carboxylase
• C. ATP synthase
• D. NADP reductase

Hint:

PEP carboxylase has a high affinity for CO$_2$ and does not show oxygenase activity, which helps C4 plants minimize photorespiration.

Answer 1

The Correct Option is B

Concept: In C4 plants, photosynthesis is spatially separated between mesophyll cells and bundle sheath cells. The initial fixation of carbon dioxide (primary carboxylation) occurs in mesophyll cells using a specialized enzyme.
Step 1: Understanding primary carboxylation.}
Primary carboxylation refers to the first step of CO$_2$ fixation, where CO$_2$ is incorporated into an organic compound.
Step 2: Identifying the enzyme in mesophyll cells.}
In C4 plants, phosphoenolpyruvate (PEP) combines with CO$_2$ in mesophyll cells. This reaction is catalyzed by the enzyme PEP carboxylase, forming oxaloacetic acid (OAA).
Step 3: Eliminating other options.}

• RuBisCO – Functions in bundle sheath cells during the Calvin cycle.
• ATP synthase – Involved in ATP formation, not carboxylation.
• NADP reductase – Involved in reduction of NADP$^+$, not CO$_2$ fixation.